DeLuca v. Brown (1995) is a landmark decision requiring the VA to consider functional loss from pain, weakness, fatigability, incoordination, and flare-ups when rating musculoskeletal conditions. Before DeLuca, the VA often rated based solely on range of motion measurements taken at a single C&P exam, which might not reflect the veteran's typical or worst functioning.
Under 38 CFR 4.40 and 4.45, the VA must assess additional functional loss beyond what is demonstrated on the exam table. This means if your knee bends to 50 degrees at the exam but during a flare-up it can only bend to 30 degrees, the VA should consider that flare-up limitation. The examiner is required to estimate the additional loss of range of motion during flare-ups, expressed in degrees.
To benefit from the DeLuca factors, you need to clearly describe your flare-ups during the C&P exam. Explain how often they occur, how long they last, what triggers them, and critically, how much additional limitation you experience. "My back is worse during flare-ups" is vague. "During a flare-up, I estimate I can only bend forward about 20 degrees instead of the 50 degrees I can manage on a good day, and the flare-ups happen two to three times per month and last about three days each" gives the examiner the specific information needed to document functional loss.
The Mitchell v. Shinseki (2011) decision further clarified that pain alone, without functional loss, does not warrant a higher rating. The pain must actually limit motion or function. So focus on describing what you cannot do during flare-ups, not just the pain level.
If the examiner does not address DeLuca factors in the exam report, that is a deficiency that can be the basis for requesting a new exam or appealing the decision.